1. Field of Invention
The present invention relates to a method and to an apparatus for the keeping constant of a pH of a medical liquid when draining from a container. Such a method and such an apparatus are in particular used in the manufacture and filling of medical liquids, in particular in the manufacture and filling of medical solutions such as a bicarbonate dialysis solution.
2. Description of the Prior Art
Such medical liquids are manufactured in so-called batch containers under sterile conditions from water (WFI—water for injection) and the soluble additives. The batch containers have e.g. a size of around 24,000 liters and are filled with liquid with a changing level. A gas space is located above the liquid level in the container and is in a complex interaction with the solution with respect to the chemical-physical equilibrium. The medical liquid, e.g. a bicarbonate solution, should always have a defined pH within very narrow tolerances. A stripping of CO2 from the solution into the gas space occurs as a disturbance variable in this connection, which results in an increase in pH.
To counter this, in known methods, the solution is gassed with CO2, with the CO2 being injected into the solution via nozzles in the region of the base of the batch container (reactor). The fact that pH sensors drift or show measured value differences as time passes and therefore have to be calibrated regularly in particular has an aggravating effect. However, in the case of inline sensors, this contradicts the maintenance of the sterile conditions. In this connection, an increase in the pH due to the gassing of CO2 is countered such that deviations from the desired value are determined on the basis of solution samples to be taken manually at regular intervals with a subsequent laboratory determination of the pH and thereupon the solution is gassed with CO2 in accordance with values based on experience via nozzles at the reactor base. A typical 2-point controller development of the pH thereby results over the manufacturing period. The control deviation of the pH is considerable due to the huge time requirement of analytical chemistry. The manufacturing method is correspondingly sluggish. In addition, this method of maintaining a constant pH is extremely personnel-intensive and eludes automation. The fact that pH sensors drift or show measured value differences as time passes and therefore have to be calibrated regularly in particular has an aggravating effect. However, in the case of inline sensors, this contradicts the maintenance of the sterile conditions.